1. Field of the Invention
The present invention relates to a drill bit. Drilling tools are generally classified into two types, one of them being a trepanning tool for drilling a hole with a core along the axis of rotation, and the other being a drilling tool for drilling a hole without any core. The present invention is concerned with a drilling tool which belongs to the latter type, namely a drilling tool for drilling a hole without any core left. This type of drilling tool is capable of drilling not only a through hole which extends through a workpiece but also a blind hole which has a bottom in a workpiece.
2. Description of the Prior Art
Prior to entering description of the present invention the hitherto known drill bit of the kind will be described.
FIGS. 1 and 2 show a conventional solid type drill bit. A body 1 of the drill bit comprises a shank 2 and a cutting means 3 which is formed integrally on the forward end of the shank 2. This cutting means 3 has two main cutting edges 4a and 4b at its end face. This main cutting edges 4a and 4b are defined respectively by relief surfaces 5a and 5b and rake surfaces 6a and 6b. A chisel cutting edge 8 is also defined by the two relief surfaces 5a and 5b. Marginal sections 7a and 7b are formed on the outer periphery of the forward end. The chisel cutting edge 8 has an angle which is substantially equal to the angle defined by the two main cutting edges 4a, 4b. The chisel cutting edge 8 is a continuation from the main cutting edges 4a and 4b. It is to be noted that the important features of this conventional drill bit are that the chisel cutting edge 8 is provided and that the chisel cutting edge 8 passes through the axis 0 of rotation of the bit body 1.
One drawback of such a conventional drill bit is that an increased thrust load (axial component of cutting resistance) is exerted on the drill bit. As a result, the drill bit is damaged, and the chisel cutting edge is subjected to substantial wear. The reason that the increased thrust load is produced is that the chisel cutting edge 8 extends through the axis 0 of rotation. More specifically, the relief angle of each relief surface need to be sufficiently large that the relief surfaces 5a, 5b do not come into contact with the workpiece when drilling a hole therein. The minimum relief angle is different along the cutting edge, and the closer it approaches to the center of the drill bit, the larger it should be. Theoretically it should have 90 degrees at the center. However, it is impossible to manufacture such a drill bit which has the above-mentioned relief angle. Therefore, in the case of the conventional drill bit in which the cutting edge passes through the axis of rotation, the relief surfaces come into contact with the workpiece within the extent of a certain distance from the center so that the increased thrust load is produced. Another reason that the increased thrust load is produced is that the chisel cutting edge 8 has a negative rake angle. More specifically, the chisel cutting edge 8 has the negative rake angle which is equal to about half of the edge angle .theta. of the drill bit. With this construction, the chisel cutting edge is forced into the work piece rather than cuts the same during the drilling operation, so that the chisel cutting edge 8 is subjected to the increased thrust load. This problem of increased thrust load is serious particularly with a drill bit made of a super hard alloy such as cemented carbide, because such a drill bit is liable to be damaged by the thrust load involved.
Another drawback of the above-mentioned conventional drill bit is that melt-adhesion takes place on the chisel cutting edge 8. The melt-adhesion referred to above represents a phenomenon that some cuttings or chips become soft by the heat generated during the cutting operation, and are caused to adhere to the chisel cutting edge 8 by a great cutting pressure involved and then are solidified. Particularly, where a drill bit is made of a super hard alloy, there are occasions when part of the chisel cutting edge 8 or part of the main cutting edges adjacent thereto are forcibly chipped off from the remainder when the adhered cuttings are separated from the chisel cutting edge 8. The problem of melt-adhesion arises from the fact that the drill bit has a considerable negative rake angle and that the cutting is effected at a relatively low speed. Accordingly, as far as the conventional drill bit of the type, in which the chisel cutting edge 8 passes through the axis of rotation, is concerned, it is unavoidable that the melt-adhesion takes place, thereby giving rise to the above-mentioned chipping of part of the chisel cutting edge or part of the main cutting edges adjacent thereto. A further drawback of the conventional drill bit is that it tends to shake during the cutting operation. This shaking of the drill bit arises from the fact that the opposite ends 8a and 8b of the chisel cutting edge 8 alternately serve as a temporary center of rotation. The shaking of the drill bit makes it difficult to drill a straight hole of a circular cross-section. Further, the shaking of the drill bit cause the thrust load and the torque load to fluctuate to such an extent that the cutting operation can not be properly effected and that the drill tip is damaged. Particularly, where a drill bit is made of a super hard alloy, it is quite possible that the drill bit is subjected to damage.
There is known a drill bit which is produced by a thinning method so as to eliminate the above-mentioned drawbacks of the conventional drill bit. A drill bit manufactured by an X type thinning method will be described with reference to FIGS. 3 and 4. This drill bit is formed with another pair of relief surfaces 10a and 10b adjacent to the relief surfaces 5a and 5b. The relief surfaces 5a and 5b and rake surfaces 12a and 12b define thinned cutting edges 11a and 11b, respectively. The rake surfaces 12a and 12b extend along the axis 0 of rotation so that the cutting edges 11a and 11b have a rake angle which is approximately 0.degree. with respect to the axis of rotation. This drill bit is subjected to a thrust load which is less than that of the drill bit as shown in FIGS. 1 and 2 but is still greater than an acceptable thrust load because of the fact that the cutting edges 11a and 11b pass through the axis 0 of rotation. Further, since the cutting edges 11a and 11b pass through the axis 0 of rotation, adverse effects would occur such as the melt-adhesion of cuttings to the edges and the formation of built-up edge. In addition, this type of drill bit has the disadvantage that the cutting edges 11a and 11b have a reduced strength since the forward end portion is thinned by the thinning. As a result, the heavy cutting can not be effected. Further, the total length of the cutting edges 11a and 11b is almost equal to the length of the chisel edge 8 of the drill bit shown in FIGS. 1 and 2, and they pass through the axis 0 of rotation. As a result, the shaking of the drill bit can not be prevented from occuring.
Next, other well known example of a solid type drill bit with two lines of cutting edges will be illustrated and described below.
FIGS. 5 and 6 show a drill bit which is disclosed in Japanese Patent Application Laid-Open No. 51-62475. This drill has at its forward end an oval recess 30 disposed eccentric to the axis 0 of rotation. The chisel cutting edge 31 of the drill bit is interrupted by the oval recess 30. No mention is made in this application of whether or not the chisel cutting edge 31 passes through the axis of rotation, but it appears from the drawings that it passes through the axis of rotation. Since the chisel cutting edge 31 passes through the axis 0 of rotation, it is readily understood that the above-mentioned drawbacks of the conventional drill bit shown in FIGS. 1 and 2 such as the melt-adhesion of cuttings to the chisel cutting edge 31, the formation of a built-up edge and the shaking of the drill bit are not satisfactorily eliminated.
FIGS. 7 and 8 show a drill bit of the brazing type disclosed in Japanese Patent Application Laid-Open No. 54-78587, in which a pair of cutting means 50a, 50b are secured to the forward end of the bit body 1 by brazing. The cutting means 50a and 50b are of the same configuration and are secured to the bit body 1 in such a manner that they are disposed symmetrically with respect to the axis 0 of rotation of the bit body 1. The cutting means 50a and 50b have respective curved cutting edges 51a and 51b each having an arcuate inner portion. The inner end of each of the cutting edges 51a and 51b is located on the axis 0 of rotation of the drill bit body 1.
A drill bit shown in FIGS. 9 and 10 has two cutting means 60a and 60b of different configuration brazed to the forward end of the drill bit body 1. A cutting edge 61a of the cutting means 60a passes through the axis 0 of rotation of the drill bit body 1.
A drill bit shown in FIGS. 11 and 12 has three cutting means 70a, 70b and 70c which are brazed to the forward end of the drill bit body 1. A cutting edge 71b of the central cutting member 70b passes through the axis 0 of rotation of the drill bit body 1.
A common feature of the conventional drill bits shown in FIGS. 7 to 12 is that their cutting edges pass through the axis of rotation. This means that they have the same drawbacks as the above-mentioned other conventional drill bits have.
There is known an insert type drill bit which has cutting means in the form of an insert, the cutting means being detachably secured to the forward end of the drill bit body by fastening means such as screws. Japanese Laid-Open No. 53-15,234 discloses one of such insert type drill bits. Since the cutting edges of the inserts pass through the axis of rotation of the drill bit body, the drill bit has the same drawbacks as the above-mentioned conventional drill bits of the solid type have.
Finally, a typical example of conventional gun drill bits will be described with reference to FIGS. 13 and 14. This gun drill bit has a single cutting edge 80 which passes through the axis 0 of rotation of the drill bit body. Therefore, the gun drill bit has such drawbacks as increased thrust load, the melt-adhesion of cuttings to the cutting edge 80, the formation of a built-up edge, and the chipping of part of the cutting edge from the remainder, as is the case with the above-mentioned solid type drill bits with two cutting edges.